Flux-trap research and testing nuclear reactor



March 1, W66 c. F. LEYsE ETAL 3,233,107

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March l, 1966 c. F. LEYsE ETAL 3,238,107

FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR Filed April 5, 1963 l5 Sheets-Sheet 5 Car? E Legse Oscar Eigert' ig'. 5.

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INVENTORS.

I March l, 1966 c. F. LEYsE ETAL FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR 15 Sheets-Sheet 7 Filed April 3, 1963 /o ogy' L I "fir/oo fag. 13.

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March 1, 1966 C. F, LEYSE ETAL 3,238,107

FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR Filed April 3, 1963 15 Sheets-Sheet 8 Oscar JI Eigert- Byron Rleonard, In INVENTORS` BY g March l, 1966 C, F, LEYSE ETAL 3,238,107

FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR Filed April 3, 1963 15 Sheets-Sheet 9 FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR Filed April 5, 1963 15 Sheets-Sheet 10 Byron RLe0`nard,J'n INVENToRs.

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March l, 1966 c. F. LEYSE ETAL FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR Car? E @scar JE yrow IY. Zeal/lg( Filed April 3, 1963 March l, 1966 C, F, LEYSE ETAL 3,238,107

FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR Filed April 5, 1965 15 Sheets-Sheet 14.

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March 1, 1966 C, F, EYSE ETAL 3,238,107

TRAP RESEARCH AND TESTING NUCLEAR REAGTOR FLUX- l5 Sheets-Sheet 15 Filed April 5, 1963 rd, J".

INVENTORS rm HL ema United States Patent O 3,238,107 FLUX-TRAP RESEARCH AND TESTING NUCLEAR REACTOR Carl F. Leyse, Chesterfield, Mo., Oscar J. Elgert, Sunnyvale, Calif., and Byron H. Leonard, lr., St. Louis, Mo., assignors to Internuclear Company, a corporation of Missouri Filed Apr. 3, 1963, Ser. No. 270,272 13 Claims. (Cl. 176-62) This invention relates to research and testing nuclear, or neutronic, reactors and more particularly lto ilux-trap nuclear reactors primarily for use in research and development.

Nuclear reactors and their construction and operation are well known and specific details of the theory and essential characteristics of nuclear reactors are set forth in prior art publications including, among others, (1) U.S. Patent No. 2,708,656, issued on May 17, 1955, to E. Fermi and L. Szilard, (2) Experimental Production of Divergent Chain Reaction, E. Fermi, American Journal of Physics, vol. 20, No. 9, December 1952, (3) Science and Engineering 4of Nuclear Power, C. Goodman, Addison-Wesley Press, Inc., Cambridge, Mass., vol. 1 (1947) and vol. 2 (1949), (4) The Elements of Nuclear Reactor Theory, S. Glasstone and M. Edlund, D. Van Nostrand Co., Inc., New York, 1952, (5) Elementary Pile Theory, H. Soodak and E. C. Campbell, John Wiley and Sons, New York 1950, (6) Current Status of Nuclear Reactor Theory, A. Weinberg, Am. J. `of Phys., vol. 20, October 1952, lpp. 401-412, (7) Mult-igroup Methods for Neutron Diffusion Problems, R. Ehrlich and H. Hurwitz, Jr., Nucleon'ics, vol. l2, No. 2, February 1954, pp. 23-30, (8) Neutron Cross Ssections, A.E.C.U.-2040, OTS, Dept. of Commerce, and (9) the engineering test reactor utilizing the ilux-trap principle disclosed in U.S. Patent No. 2,857,324, issued on Oct. 2l, 1958.

In research and testing nuclear reactions it is highly desirable to obtain maximum neutron utilization coupled with Imaximum accessibility for experimentation in regions of high neutron flux, and the most etlective method rfor achieving this is to employ the flux-trap principle in a nuclear lreactor which is characterized by thin fuel regions adjacent to a good moderator which slows down the neutrons and causes the thermal neutron ux to peak in a region accessible for experimentation. The simplest geometry and the most effective for loop testing is an annular core surround-ing a light water island, said light water island being open to the pool and capable of accommodating for insertion therein a thimble or U-tube loop. Typical unperturbed thermal neutron lluxes in thimble or U-tube loop inserted in the light water island may be four to six times that available in the rellector of a typical solid core research nuclear reactor operated at the same power level. Said island constitutes the center ilux-trap, which is open to the pool. In a nuclear reactor utilizing the flux-trap principle the `advantage of high leakage applies also to neutron utilization outside of the reactor core, for example, in the reflector, so that high neutron uxes are provided in experimental positions in the retlector.

Although open pool flux-trap reactors can be constructed in the lower power range such as up to 5 mw., the `open pool type is limited in higher power level by several ellects such as (l) heat exchange requirements and (2) pool activity. Since the core water must enter at pool temperature, the mean temperature diierence is small while surface requirements 'are large. Since the aluminum clad fuel leads to sodium activity [Al2'7(n,a)Na24], this activity is generally removed 'by an ion exchange demineralizer, and the production of Na24 increases as the power level of the reactor increases. For example, at l0 mw., demineralizer ilow of 400 g.p.m. is required to obtain tolerance at the -surface of a pool of normal size, and demineralizing such large amounts of water requires large and expensive demineralizer systems.

The effectiveness of a research nuclear reactor utilizing the flux-trap principle can be enhanced by pressurizing the core. However, when pressurized core vessels are employed, various problems are impose on the systems, particularly as applied to operating the reactor without complicated sealing systems.

It is highly desirable to provide a relatively inexpensive research nuclear reactor with -a pressurized core where the problems associated therewith are minimized and the operations of the reactor and test loop are carried out without disturbing the pressurized core. It is also desirable to provide a nuclear reactor having simplified and elllcient reilector control and experimental facilities.

A unique feature of this invention is a relatively thin annulus of a suitable shim material, such as borated H2O, located between the reactor core and the rellector. The radius ratio of the borated shim region to the radius of the total reflector region is, for example, about 1/10 to 1/75, such as about l/l5 to 1/40, ibut preferably about l/ 20 to 1/30. Where D20 is employed in a droppable safety system, the radius ratio of the shim region to the D20 region is from about l/5 to 1/40, such as about l/lO to l/20, but preferably about l/l2 to l/l6. The use of the liquid shim provides the following advantages:

(l) The size of the system is reduced;

(2) Borated H2O can 'be used in place of borated D20 system; a thin region yof -borated H2O is about as effective as a thicker region of borated D20 and has the great advantage of not requiring a deuterated Iresin bed to adjust Iboron concentration;

(3) It is compatible with both the more conventional solid reflector and shim-safety rods as well as a droppable D20 safety system;

(4) The extent to which `shim controls perturb the neutron iluxes in the reflector test holes is minimized; and

(5) A simple `on-ofl control, similar to that required by control Ablade, of boric acid concentration by injecting pure demineralized water or a saturated boric acid solution, is adequate for shim control.

Another unique feature of this invention is the particular construction of the Ireactor with respect to the ffact that all test holes are open to the pool thereby -to facilitate the insertion of experimental facilities therein without the problem of pressure sealing and to make it possible for experimental test loops to Ibe easily inserted therein and to operate independently of other systems of the reactor. The split top head of the core permits refueling without removal of the control test loop.

Contemplated to be within the scope of this invention are several embodiments of our flux-trap research and testing nuclear reactor, each embodiment of the nuclear reactor comprising basically a vertical cylinder ux-trap reactor with a light water central island constituting a test hole, o-r well, open to the pool, a light water lcooled aluminum plate type core contained in a slightly pressurized annular vessel, a thin -borated light water shim control yregion just outside said vessel, rfollowed by the reflector assembly. In one embodiment there is provided, as part of the reflector assembly, an inner reflector region of D20, which is dropped to provide safety control. In another embodiment there is provided, as part yof the `reflector assembly, instead of the inner reflector region of D20, an inner reflector region of beryllium and conventional safety rods in combination with a thin 'borated light water shim control region just outside said vessel, followed Iby the aforedescribed 'reflector assembly. In lall embodiments an outer ygraphite reflector is provided. The nuclear reactor of each embodiment is capable of operating7 at 20 mw. power level with the addition only of cooling tower capacity and at 30 mw. power level with non-uniformly loaded fuel and additional coo-ling tower capacity.

An object of this invention is to provide a simplified research nuclear reactor utilizing the flux-trap principle comprising a pressurized vessel having positioned therein a core containing materials fssionable by neutrons, primarily of thermal energy, or atomic fuel, or nuclear fuel, where the operations thereof can be carried out without disturbing the pressurized core; and the reactor has simplified and efficient reflector control and experimental facilities.

Another object of this invention is to provide a relatively inexpensive research nuclear reactor incorporating the flux-trap principle for obtaining relatively high thermal neutron fluxes in the center test hole as well as in the reflector test holes.

Another object of this invention is to provide a research flux-trap nuclear reactor including a pressurized core vessel surrounding a non-pressurized island constituting a test hole.

Another object of this invention is to provide a rellector having a plurality of test holes which are open directly to the pool.

Another object of this invention is to provide a split top cover for the core vessel which permits pressurizing the core for high power operation but which can be removed for refueling the reactor without disturbing the test loops inserted in the central test hole.

Another object of this invention is to provide a liquid shim region of borated H or of borated D20 in a relatively thin annulus located between the reactor core and the reflector.

Another object of this invention is to provide a heavy water reflector which can be dropped for safety control, preferably having scram valves above the cor-e for ease of maintenance in a relatively low irradiation field but still being sufficiently close to minimize gas flow resistance so as to enhance scram speed.

Another object of this invention is to provide experimental test loops or test assemblies which can be inserted in the non-pressurized core island constituting a test hole therein and in the non-pressurized test holes in the reflector thus facilitating operational handling, maintenance and safety; since these test loops do not require penetration of the pool wall below the reflector, they can be easily replaced.

Another object of this invention is to provide a liquid shim of borated H20 or of borated D20 in a relatively thin annulus located between the reactor core and the reflector, said reflector comprising either (1) a droppable D20 region whose size is sullicient for safety control, employed in combination with graphite in the outer reflector so as to reduce cost, or (2) other suitable reflectors in place of D20, for example beryllium, in conjunction with graphite, employing conventional control rods.

Other objects and features of this invention will become apparent from the following detailed description which is illustrative and non-limiting.

Like numerals designate like components.

FIGURE 1 is a graphical representation of the average radial flux distribution within the reactors of this invention showing thermal neutron flux and fast neutron flux plotted against radius, or radial distance, in centimeters from the center line of the center test hole, said center line being zero, sodium having been chosen only to represent a typical absorbing material, each of said reactors having a clean shim region.

FIGURE 2 is a graphical representation such as is that of FIGURE 1 and differs from FIGURE l in that the values were obtained with a poison shim region.

FIGURE 3 is a graphical representation of the axial thermal neutron flux and fast neutron flux distributions at the center of the core of the reactors of this invention.

FIGURE 4 is a graphical representation of the axial thermal neutron flux and fast neutron flux distributions of a sodium-filled test loop in a test hole through the reflector of each of the reactors of this invention.

FIGURE 5 is a View in elevation, partially in section and partially in block schematic, of one embodiment of the nuclear reactors of this invention, taken along line 5-5 in FIGURE 6.

FIGURE 6 is a top plan view of the reactor of FIG- URE 5 showing particular layout features.

FIGURE 7 is a vertical sectional view taken on line 7 7 of FIGURE 8 showing one embodiment of annular core.

FIGURE 8 is a sectional View taken on line 8 8 of FIGURE 5 showing each embodiment of the fuel elements.

FIGURE 9 is a top plan View of a fuel element in one embodiment of the annular core.

FIGURE 10 is a flow diagram of the coolant system.

FIGURE l1 is an enlarged detail of the section within A of FIGURE 9.

FIGURE l2 is a side elevational view of the fuel element in FIGURE 9.

FIGURE 13 is a top elevational View of the fuel element in FIGURE 9.

FIGURE 13a is a top elevational view of end fuel plate of the fuel element in FIGURE 9.

FIGURE 14 is a top plan View of a fuel element in another embodiment of the annular core.

FIGURE 15 is an enlarged detail of the section within B of FIGURE 14.

FIGURE 16 is a slide elevational view of the fuel element in FIGURE 14.

FIGURE 17 is a View in the direction of line 17-17 in FIGURE 14.

FIGURE 18 is a partial sectional view in perspective including the pressurized reactor core vessel, the control region, and the reflector assembly.

FIGURE 19 is schematic of the liquid shim control region flow diagram.

FIGURE 20 is a schematic of the droppable liquid D20 reflector flow diagram.

FIGURE 21 is a front elevational view partly in section of a thimble test loop.

FIGURE 22 is a view in elevation, partially in section and partially in block schematic, of another embodiment of the nuclear reactors of this invention, taken along line 22-22 in FIGURE 23.

FIGURE 23 is a top plan View of the reactor of FIG- URE 22 showing particular layout features.

FIGURE 24 is a sectional view taken on line 24-24 of FIGURE 22 showing each embodiment of the fuel elements in the embodiment of the reactor of FIG- URE 22.

More specifically, nuclear reactor 2, including annular pressurized vessel 4, reflector assembly 5 including droppable fluid, preferably liquid D20, reflector portion 6 and graphite portion 8, is positioned in main pool vessel 12, and said reactor 2 is covered, during operation thereof, by demineralized light Water 14. Surrounding reactor 2 is a barytes concrete shield 16, and pool water 14 and shield 16 constitute the radial reactor shield, the density of said concrete shield 16 being 3.4 grams/ cubic centimeter. Said pool water 14 constituting the reactor pool in said main pool vessel 12 is 33 feet deep. Said annular pressurized vessel 4 is of aluminum, approximately 11% inches inside diameter and approximately 19% inches outside diameter at the annular reactor core section, said reactor core 42 being positioned Within said annular pressurized vessel 4, said annular core 42 containing the atomic fuel, or material tissionable by thermal energy, hereinafter more fully decribed. The inner vessel wall 18 of said annular pressurized vessel 4 extends straight upward above said core 42 approximately 6 feet to provide maximum experimental space in the centrally positioned light water island 20 constituting the central test well, or hole, hereinafter more fully described. The outer vessel Wall 22 of said annular pressurized vessel 4 extends upward approximately 5 feet above said core 42 in a `straight section and then ares out to a diameter of approximately 26 inches for the remaining distance. Said five-foot straight section 24 permits the removal and the replacement of the liquid shim control region 26, hereinafter more fully described, and the so-formed larger diameter section 28 facilitates refueling. Split top head 311 is provided to seal positively said annular pressurized vessel 4 while providing an opening permitting light water island 20 to open into said pool water 14 and to permit its cornplete removal from around central test loop 32 inserted in island 20 so as not to interfere with refueling. Below core 42 vessel walls 18 and 22 extend straight `downwardly for approxi-mately two feet and then are outwardly and extend horizontally to form an outlet coolant plenum and a support 34 for the reector assembly 5. Reactor assembly 2 is supported by support legs 36 below the reflector plenum. Inlet coolant water enters approximately 41/2 feet above core 42 through reactor coolant inlet nozzle 38. The long, straight section 38) of reactor vessel 4 above core 42 establishes uniform radial flow distribution to the top of core 42. Aluminum outlet coolant nozzle 40, approximately 14 inches in diameter, extends horizontally from the bottom plenum. Each of nozzles 38 and 4t) are flanged, as at 44 and 46, respectively, and connected to stainless steel primary coolant pipes 48 and 50, respectively, within pool vessel 12 in pool water 14. Of particular note is the fact that maximum use of stainless steel equipment in the primary coolant loop 52 including nozzles 38, 40, 48 and 50 minimizes corrosion and thereby reduces the demineralizer ow requirement.

Each fuel element of annular core 42 may be one of two embodiments.

The embodiment of the fuel element 70 has twentyve curved fuel plates 54 each of varying circumferential length spanning the distance between radial side plates 56 and 58 of each element. Each fuel plate 54 consists preferably of a 20 mil section 60 of U-Al alloy (29% U in Al, uranium being 90% enriched) clad on both sides with mils of aluminum 62. Each Water gap 64 between two adjacent fuel plates 54 is 75 mils thick and 50 mil water gaps 66, 68 are left beyond inner and outer 4fuel plates 54, 54. The active fuel length of each plate 54 is thirty-six inches. Fuel element 70 consists of twentyfive of said curved fuel plates 54, and eighteen of said fuel elements 70 form annular core 42. Each of fuel elements 70 is supported by spider 77 having radial ribs S4 spanning annular core 42 and grooved to form channels 74 for side plates 56 and 58 and each fuel element 70 is held down only by the ow of pressurized light water flowing through said pressurized vessel 4. Each element is aligned at its top portion by pin 78 resting in slot 80 in island wall 18. Pin 78 is toggled to comb 82 of fuel element 70, so as to point in either of two direction-s, downwardly when at the bottom of element '70 and inwardly when at the top of element 78, thereby allowing each fuel element 70 to be individually inverted. Each fuel plate 54 is fitted into and positioned in a longitudinally extending groove 76 inside plate 56 and a longitudinally extending groove 84a in side plate 58. Attached to side plates 56 and 58 is lifting bar 86, thereby enabling easy removal of each fuel element 70.

A second embodiment of the fuel element having the same thermal and nuclear characteristics as fuel element 7i? is fuel element 90 having a plate curvature conforming to the involute of the circle describing the inner boundary, viz. inner Wall 18, of annular core 42. Involute fuel element consists of 16 fuel plates 92. Involute fuel element 90 is supported at the bottom by a shoulder (not shown) on the wall 22 of pressurized vessel 4 and on wall 18, and said shoulder is notched in forty plates to accept each of the twenty positioning members 94 on either end of each of fuel elements 90. To invert each of the twenty fuel elements 90 at once for the reason that each of side plates 96 and 98 would then curve in the opposite direction, there are provided alignment pins 100, shown to be eight in number, and positioning members 94 then match the second set of twenty notches on said wall 22. Each fuel plate 92 is fitted into and positioned in a longitudinally extending groove 102 in side plate 96 and a longitudinally extending groove 104 in side plate 98. Attached to side plates 96 and 98 is lifting bar 106, thereby enabling each removal of each fuel element 90. With respect to involute fuel element 90 and each of the fuel plates 92, the characteristics thereof are shown by the dimensional values of the component parts of such an element 90 as can be used, by way of a specific example, in a nuclear reactor of this invention, as follows:

Immediately surrounding reactor vessel 4 is liquid shim control region 26 in an approximately 5% inch thick annulus between vessel 4 and D20 reflector tank 112. Shim region 26 is composed of a ring section 114 containing 1/2 inch diameter tubes 116 provided for lflow of borated light water. Each of sleeves 108, 11S, 120 and 122 of said section 114 in region 26 are removable and replaceable with another liquid shim section or curved control blades. Borated water, or boric acid of controlled variable concentration, ows from the upper plenums through pipes 124, one for each sleeve, into headers (not shown), one for each sleeve, and back through tubes 116 and discharges to return headers (not shown) at about F. A thin region 26 of borated light water is almost as effective as a thicker region of borated D20 and has the great advantage of not requiring a deuterated resin bed to adjust boron concentration nor another external D20 ow circuit. A 50% saturated boric acid solution in light water at room temperature provides 0.95 AK control, which is more than adequate for 500 Mwd operation. Simple on-off control, similar to that required by control blades, of boric acid concentration by injecting pure demineralized water or a saturated boric acid solution, is adequate foishim control.

The shim system includes the shim region reflector 26, heat exchanger 126, expansion tank 128, water injection pump 130, lboric acid feed and mixing tank 132 and pump 134. IFlow of the borated solution is from tank 132 to the inlet plenum at the top of and above the shim region reflector 26, thence in parallel iiow through tubes 116, to the outlet plenum and through outlet pipes and headers (not shown) also located at the top `of and above said shim region reflector 26, through heat exchanger 126 and back to tank 132. Said liquid shim region 26 is pressurized to 75 p.s.i.g. to eliminate the net production of radiolytic gas. Shim heat exchanger 126 removes 96 kw. maximum heat generated before the solution enters expansion tank 128 which assures thorough mixing of liquids or removal of excess fluid during dilution operations. Excess gases generated by radiolytic decomposition of the boric acid solution are removed from the system through tank 128. Centrifugal pump 130 forces the sol'ution back to shim region 26 at the rate of 10 gpm. Heat exchanger 126 is designed for 10 atm. pressure. All parts of heat exchanger 126 outside the pool are fabricated of 316 stainless steel, the secondary water being on the shell side fabricated of `carbon steel. Shim region rellector 26 worth is altered by either diluting with demineralized water and discharging excess fluid to a hold- Iup tank (not shown) or by injecting saturated boric acid solution into expansion tank 128 while discharging the excess dilute acid to the holdup tank. The solution in the holdup tank is monitored for activity and discharged to the sewer or hot waste depending on activity level. ln the shim control system are included manually operated valves 37, 65 and 39, solenoid valves 41, 43, 45, 47, 49, 51, 53, 55, 57 and 59, spring loaded check valves 61 and 63, demineralizer 67, pump 69, and N2 source 71.

Immediately outside shim control region 26 is heavy water (D20) reflector 6.which is dropped for safety control, said heavy water being contained in annular 3-co1npartment aluminum vessel 136 having an outside diameter of 38 inches and forming a 9 inch thick safety rellector region around core 42. Tank 136 extends 6 inches above and 30 inches below core 42. The bottom 24 inches of vessel 136 is expanded to 60 inches outside diameter to form a plenum into which the heavy water is dropped. Perforated plate 138 near top of core 42 provides ilow of D20 for cooling and permits the rellector to be dropped from the top portion about core 42 only. Two by-pass scram lines 140, 142 extending from the top of the drop region to the plenum are normally closed by four solenoid valves 1, 3, 7 and 9 in parallel, each of said Valves 1, 3, 7 and 9 being positioned on support spider 144 above reactor core 42 and constituting scram valve assembly 146 to enable servicing in a low radiation field. Scram is initiated by opening said spring loaded solenoid valves 1, 3, 7 and 9 in said by-pass lines 140, 142, said lines 140, 142 short circuiting jet eductor 148 and connecting the gas space in the plenum with the region to be voided. The D20 drops out of the reflector region in about one second, the major resistance to reflector drop being the gas ilow. The negative worth of safety rellector 6 is calculated to be at least 0.16 AK which is adequate to shut the reactor down under any fault condition.

In the operation of the D20 reflector water coolant ilows from vessel 136 to the D20 heat exchanger 152 and back to the lower plenum. Water jet eductor 148, in line 150 'from the top of rellector Vessel 136 to the plenum, raises the D20 into the reflector region 6 and provides flow from the top of tank or vessel 136 to the plenum through line 150. Approximately 800 kw. of heat is generated in reflector 6 when reactor 2 is operating at l mw., about 15% of this heat being generated in the upper compartment of tank 136. The heavy water is cooled by withdrawing about 10 g.p.m. from the upper compartment and 60 gpm. from the lower compartment and passing the 170 KF. combined flows into heat exchanger 152 where the temperature is reduced to 110 F. The deuterium passes, by means lof pump 33 from the lower plenum of tank 136 through lheater 160 to recombiner 154. The radiolytic deuterium-oxygen-nitrogen gas mixture is heated, and the deuterium is catalytically recombined in catalytic recombiner 154 into heavy water. An after cooler 156 reduces the gas temperature to 212 F. In the system for the D safety rellector 6 are included spring loaded check valves 11, 13 and 15, solenoid valves 17 (from the N2 source), 19 (from the 02 source), 21, 23, 23a and 25, manually operated valves 27 and 29, pumps 31 and 33, and demineralizer 35. As is readily apparent, tank 136 can be rapidly filled or emptied of D20, as operating conditions require. Upper support spider 144 is positioned approximately 11 feet above core 42 and is held in position and rests on support lugs 73 embedded in the wall of the main pool vessel 12, said spider 144 forming a support for said scram valves 1, 3, 7 and 9, other experiment leads, etc. Pool water 14 can be lowered to the spider level to facilitate servicing of said scram valves and other equipment.

Through said D20 rellector 6 are provided directly open to pool water 14 seven vertical experimental test holes or wells 75, 77, 79, 81, 83, 85 and 87 to Iaccommodate test loops or capsules. When each of said holes or wells is filled with light water, the average thermal neutron flux is about 1.2 1014 n./cm.2sec. An eighth vertical hole 87a through said D20 reflector is provided for a regulating rod (not shown).

Surrounding safety rellector 6 is the annular ring graphite reflector 8 which is 101/2 inches thick and 48 inches high and is of impervious reactor grade graphite in eight spaced-apart sections 89, 91, 93, 95, 97, 99, 99a and 101 which are adequately cooled by natural convection of pool water 14. Said graphite rellector -8 reduces the amount of D20 required while still extending the region of relatively high neutron fluxes for potential experimental positions. Each of said graphite block sections 89- 101 can be provided with holes, such as test hole 103 having a test loop 105, for experiments. Unperturbed thermal neutron fluxes up to approximately 9 1013 n./cm.2-sec.

are available in said graphite rellector 8.

The entire reactor assembly 2 is positioned in vessel 4 being l0 feet in diameter by 33 feet high filled with demineralized pool water 14, said vessel 4 being steel lined, said pool water 14 being approximately 241A. feet above core 42 4and providing a biological shield therefor. Annular ring 16 of baryates concrete, S4 inches thick, ex tends upwardly 11 feet above core 42 centerline, said rellector assembly 5 and pool water 14 providing adequate thermal shielding for said ring 16. Radiation dose rates at all surfaces which can be approached by personnel are l mrem./ hr., or less, from direct radiation.

The main experimental test hole is centrally positioned light water island 20 open to pool 14 and constituting the center ux-trap. U-tube test loop 32, constituting a central test loop, of any diameter up to an outside diameter of approximately 11 inches can be inserted in island 20, the top flanges 107, 109 being below the surface of pool 14, and the surface of pool 14 can thereby be lowered below said top flanges 107, 109, 117 to enable removal or insertion of experiments from or in said loop 32. Loop removal well 111 is provided in the bottom of pool 14 to facilitate removal of loop 32, thereby permitting removal thereof without emptying pool 14 or removing other experiments. To remove U-tube loop 32 pool 14 is lowered to the top of reactor vessel 4, each of leg 113 and of leg 115 of loop 32 is disconnected at flange 117, which is preferably less than 111/2 inches, and at flange 109, U- tube 121 is lowered into well 111, U-tube 121. is rotated to clear reactor 2, and U-tube 121 is then raised out of pool 14. Also, thimble type loop 125 instead of a U- tube loop can be inserted into island 20.

Likewise, each of test holes 75, 77, 79, 81, 83, 85 and 87 is suitable to receive capsule experiments, thimbletype loop 125 and U-tube loop 32.

Graphite rellector 8 has therein test hole 127 having sleeve 131 to receive sleeve 139 to receive experiments. Each of said graphite sections 89-101 may be aluminum clad and is supported by perforated rellector grid 141 welded onto wall of 136. Integral with grid 141 is rellector retainer wall 143.

Reactor cooling system 145 is designed to remove 15 mw. thermal power from reactor 2, and pertinent data therefor is as follows:

TABLE I Reactor cooling system Reactor power mw Primary loop power mw 13.5 D power mw 0.8 Pool heat mw 0.65 Shim reflector heat mw 0.05 Operating pressure p.s.i.a 75 Number of primary heat exchangers 2 Primary heat exchanger area, each ft.2 3,680 Core coolant temperature:

Inlet F 110 Outlet F-- 133.5 Primary ow rate g.p.m 4,000 Secondary ilow rate g.p.m 6,400 Number of primary pumps 2 Primary pump motor rating, each H.P 75 Number of secondar pumps 2 Secondary pump motor rating, each H.P 100 Secondary coolant temperature:

On tower F-- 104 Off tower F 88 Design wet bulb temperature F 80 The primary cooling system 147 is normally operated as a closed, slightly pressurized system at 15 mw. and

pertinent data therefore is as follows:

Y TABLE II Primary cooling system or core heat transport dam Reactor power mw 15 Core:

I.D. in 12.50

O.D. in 18.16

Height in 36 Volume l 80.44 Heat transfer area ft.2 544 Coolant ow area ft.2 0.542 Coolant ow rate g.p.m 4000 Core coolant velocity ft./sec 16.5 Coolant temperature:

Inlet F 110 Outlet F 133.5 Maximum surface temperature F 240 Core pressure:

Inlet p.s.i.a 75

Hot spot p.s.i.a 69

Outlet p.s.i.a 65 Saturation temperature:

Inlet F 307 Hot spot F 300 Outlet F-- 297 Nominal burnout power at hot spot mw 100 Power density:

Average mw/l 0.187

Maximum mw/l 0.518 Max/ave. power distribtuion:

Radial 2-2 Axial 1.26

Over-All 2.77 Hot channel factors:

Bulk coolant temperature 1.3

Coolant film temperature 1.5

Annular core 42 consists of 18 circumferential curved plate fuel assembly 70 or 20 involute curved plate fu'el assembly 90. The maximum-to-average power ratios 1n the axial and radial directions (with poisoned shim) are 1.26 and 2.2, respectively. Hot channel factors of 1.3 for the bulk coolant temperatures, and 1.5 for the lm temperatures are used. At the specified ow rate of 4000 g.p.m. slightly pressurized reactor 2 can be operated at 15 mw. to provide the desired thermal neutron flux.

Maximum surface temperature at the hot spot is 240 F. and saturation temperature is 300 F. The nominal margin to heat-transfer burn-out is about a factor of seven in reactor power. Reactor 2 also is capable of operating open to pool 14 at powers up to 12 mw. Reactor power can be increased to at least 20 mw. with added secondary cooling tower capacity. With graded fuel and additional secondary cooling tower capacity, reactor power can be increased to at least 30 mw. Coolant ows at a velocity of 16.5 ft./sec. in the channels through core 42, and the pressure drop across core 42 is 11 p.s.i. The pressure drop from core 42 to the primary pumps 151, 153 is `about 5 p.s.i., and the pressure at the primary pump suction is 40 p.s.i.a. for closed loop operation. Primary heat exchangers 155, 157, each having 2000 gpm. heat exchange capacity, is a tube and shell exchanger using one shell pass and two tube passes. The primary, demineralized coolant ows inside the tubes and all surfaces in contact with the primary coolant lare of 304 LC stainless steel. The shell side material, in contact with the secondary coolant, is carbon steel. Data for each heat exchanger 155, 157 is as follows:

TABLE III Primary heat exchanger data Number Shut down emergency pump 159, rated at 300 gpm., is provided. The pump impellers of pumps 151, 153 and 159 are of 304 stainless steel, and the casings of pumps 151, 153 and 159 are of cast iron. Loop pressure is maintained by compressed air or nitrogen admit- -ted to standpipe 161 pressurized at 75 p.s.i.a. by pressure regulating valves 163, 165, and a constant gas discharge to the stack (not shown) is maintained to sweep off radiolyti'c gases. Liquid level is controlled between high and low level limits.

For water cleanup, a flow of 50 g.p.m. is provided to maintain primary coolant purity to about 1 ppm. solids. Mixed bed demineralizer units 167, 173, l0 micron lters 169, 1711 and conductivity cells, not shown, are provided for reactor 2. During open primary loop operation, all demineralizer ow is sent directly to pool 14, thereby insuring flow from pool 14 into the open primary system and maintaining pool surface activity low. During closed loop operation primary demineralizer 167 flow is directed back to the inlet side 0f each of primary pumps 151, 153; the respective flows to demineralizers 167, 173 are shut off during regeneration or replacement of resin.

All primary loop piping outside of pool 14 is 304 LC stainless steel. Outlet pipe 175 from core 442 to pumps 151, 153 is l4-inch schedule 40 pipe, and inlet pipe 177 to lcore 42 is 12-inch schedule 40 pipe. Flow orices (not shown) are provided in the primary coolant and shutdown system pipes. 

1. A NUCLEAR REACTOR COMPRISING IN COMBINATION, A MAIN VESSEL, AN ANNULAR PRESSURIZED VESSEL POSITIONED IN SAID MAIN VESSEL, A CORE CONTAINING MATERIAL FISSIONABLE BY NEUTRONS OF THERMAL ENERGY, SAID CORE BEING POSITIONED WITHIN SAID ANNULAR PRESSURIZED VESSEL, A REFLECTOR ASSEMBLY POSITIONED INSAID MAIN VESSEL, SAID REFLECTOR ASSEMBLY BEING POSITIONED ABOUT SAID PRESSURIZED VESSEL, SAID REFLECTOR ASSEMBLY INCLUDING AT LEAST ONE OF A DROPPABLE FLUID REFECTOR MATERIAL AND A SOLID REFLECTOR MATERIAL, MEANS FOR CONTROLLING SAID REACTOR, SAID REACTOR CONTROL MEANS INCLUDING AT LEAST A FLUID CONTROL MATERIAL, SAID REACTOR CONTROL MEANS BEING BETWEEN SAID PRESSURIZED ANNULAR VESSEL AND SAID REFLECTOR ASSEMBLY, MEANS FOR PASSING COOLANT THROUGH SAID MAIN VESSEL, AND MEANS FOR PASSING PRESSURIZED COOLANT THROUGH SAID ANULAR PRESSURIZED VESSEL, WHEREBY SAID CONTROLS AND SAID REFLECTOR ASSEMBLY ARE NON-PRESSURIZED AND SAID CORE IS PRESSURIZED. 